1. Field of the Invention
The present invention relates to a system and a device for reducing confluence noise which are used in a two-way CATV (cable television) system.
2. Description of the Related Art
A two-way CATV system uses a certain frequency band for downward signals directed from a center (headend) to terminals (users), and uses a different frequency band for upward signals directed from terminals to the center. These signals are transmitted through coaxial cables or optical fiber cables to achieve two-way communication. In this configuration, upward signals are mixed with noises as these signals travel to the center. Such noises are called confluence noise, and measures for reducing the confluence noise need to be devised.
FIG. 14 is an illustrative drawing showing a related-art CATV system.
The CATV system of FIG. 14 includes a center 201, a two-way amplifier 202, splitters 203, and terminals 204. The center 201, the two-way amplifier 202, and the splitters 203 are connected via a main transmission line. A plurality of two-way amplifiers 202 are generally provided at predetermined intervals along the main transmission line, thereby amplifying upward and downward signals transmitted through the main transmission line. The splitter 203 may be implemented by using 8 branching configuration. Branch transmission lines connect between the splitters 203 and the terminals 204.
Upward signals from the terminals 204 to the center 201 may be carried through a frequency band with a range between 10 MHz and 55 MHz, and downward signals from the center 201 to the terminals 204 are carried through a frequency band with a range between 70 MHz and 400 MHz. Modulation schemes based on phase modulation such as BPSK, QPSK, and QAM are generally used.
Transmission speed may be 160 kbps, 320 kbps, 640 kbps, and 2560 kbps, or 256 kbps, 1544 kbps, and 3088 kbps. The CATV system as described above may be utilized to implement an on-demand CATV system for supplying moving-picture information in response to user requests, a cable-telephone system for effecting communication between users, a system connected to a switch network to provide Internet services, etc.
Downward signals transmitted from the center 201 to the terminals 204 are split at the splitters 203 to go from the main transmission line to the plurality of branch transmission lines, and travel through the branch transmission lines to reach the terminals 204. Noises mixed with the signals along the branch transmission lines can be a cause of C/N reduction at the terminals 204, but can never cause a significant problem to the system.
On the other hand, upward signals from the terminals 204 to the center 201 merge at the splitters 203 as they go from the branch transmission lines to the main transmission line. Noises that are mixed into the signals at the terminals 204 or along the branch transmission lines cover a wide range of frequencies including the signal frequency band, and are summed at the splitters 203 as they merge, ending up being a cause of a significant C/N reduction. In order to obviate this problem, for example, a scheme is devised to convert upward signals having a frequency band the same as a wireless frequency band into signals having a different frequency band.
When a terminal 204 is disconnected at a time when a user moves out, for example, the branch transmission line ends up having an open end. The open end serves as a noise source by generating reflection noises, so that noises without signals may enter a splitter 203 via the branch transmission line. In other cases, upward signals may include large noises because of malfunction of the terminals 204. In such cases, the terminals 204 act as noise sources. When a plurality of noise sources are present, generated noises are summed at the splitters 203, thereby significantly reducing the C/N of the upward signals. In the worst case, the center 201 cannot detect received signals, resulting in a system shutdown.
Accordingly, there is a need for a scheme which can reduce confluence noise.